The Glass Surface and Ways of Its Modification
Ferdinand Trier, Munich University of Applied Science, Germany
Ulrich Ranke, Ormo Print GmbH, Germany
1. Introduction
The material glass is used in a variety of products. In many cases, the
properties of a glass product shall be optimized. This can be done by changing the
chemical composition of the glass. Some effects can also be achieved by changing
the surface of a glass product.
2. What is a glass surface?
The glass surface seems to be an abrupt change between solid glass (which
is defined by its chemical composition) and the surrounding air. A closer investigation
of the glass surface gives a more detailed view, the abrupt border is vanishing and
different zones of transition between glass and environment are appearing.
In the core of a soda-lime glass there is a network of silicon dioxide molecules.
Between these, calcium and sodium ions are embedded (fig. 1).
Fig 1: Scheme of a soda lime glass [1]
If we approach from bulk of a glass to the surface, the water content of the
glass is rising, because glass is attracting water from the atmosphere. Water is able
to break some of the connections in the silicon dioxide network and further diffuse
from the surface and micro-cracks into the glass bulk (fig. 2).
Fig. 2: Typical glass surface before coating
The sodium content near the surface may differ from the bulk material,
because sodium is volatile under conditions of the heat treatment of the glass, so it
may vaporize in some regions of the glass, but may also condense in other regions.
At the point, where the network of silicon dioxide ends, the network is
continued with chemisorbed water molecules (fig. 3). This chemically absorbed water
film goes over in a physically absorbed water film, which also collects various
adsorbates from the atmosphere, like organic molecules and dirt particles.
Fig. 3: Sodium-silicate glass with water film
The condition of the glass surface depends in a high degree from the history of
the glass. It is already changed at the manufacturing, but also while the storage of
the glass. Everybody who is concerned with glass should be aware of this fact.
3. Properties how to change the properties of a glass surface
Modifications at a glass surface can be classified as following:
o
methods, which generate a new surface by removing material
from the original surface
o
methods, which do not form a new surface, although the original
surface is modified by exchange of material
o
methods, which generate a new surface by adding additional
matter to the original surface
For these methods, there exists a variety of methods (fig. 4 )
Different Ways for Surface Modifications
Removal of
material from
surface
Modification by
material
exchange
Addition of
material to surface
Mechanical:
• Grinding
• Polishing
Diffusion:
• Glass staining
• Chemical
toughening
From plasma:
• Ion plating
• PECVD
Chemical:
• Etching
From gas phase:
• Sputtering
• PVD
• CVD
From liquid/solid
phase:
• Paint
• Ceramic coatings
• Au/Pt/Ag
Coatings
• Sol-gel
• Hybrids
Fig. 4: Ways for surface modification - overview
4. Modification by removing matter from the original surface
4.1 Mechanically ablating methods
At a mechanically ablating process, little particles are broken out of the glass
surface by the impact with a grinding matter. Usual techniques are grinding, polishing
and blasting. Mechanically ablated surfaces always become rough, but the grade of
roughness can be well controlled, so that even optically smooth surfaces are
attainable. Grinding methods are necessary, in case the shape of the glass part must
be defined very exactly, e.g. for optical parts.
Blasting is often used to get a surface with a certain roughness, without
special attention to the mechanical precision (fig. 5). A probable disadvantage of a
blasted surface is the high number of micro-cracks, which are generated at the
process. Micro-cracks are suspected to weaken the mechanical strength of the part.
On the other hand, surface frosting by blasting is typically cheaper than by etching.
Fig 5: SEM picture of a blasted glass surface
4.2 Chemically ablating methods
Ablating by etching will typically have a smoothening effect to the surface. This
polishing is often the motivation for the procedure. Another motivation is to replace
the original surface, which is probably undefined, soiled or affected with micro-cracks
with a newly created surface. The effect is a surface cleaning.
It is also possible to create a rough surface by etching. With acid frosting little
crystals are settling at the glass surface and stop the acid attack at the covered
areas, while the etching is between the crystals is not obstructed. The surface
structure, which is generated by that, is an image of the crystals (fig. 6 ).
Compared to blasted surfaces, acid frosted surfaces have a very fine structure
and a smooth touch.
Fig. 6: SEM picture of an acid frosted surface
5. Modification of the existing glass surface
Behind this term hide two traditional techniques, the glass staining and
chemical toughening. Also newer techniques, which influence the surface energy of
glass, belong to this category.
5.1 Glass staining
For glass staining, silver is brought in the glass over a surface diffusion
process. The yellow to orange-brown colour is generated by the formation of silver
clusters. The diffusion process is conducted at high temperature for several hours
from a silver containing paste, which was applied to the glass.
5.2 Chemical toughening of glass
Chemical toughening of glass is also a diffusion process. Sodium ions, which
are present in the glass are partially replaced by potassium ions. Due to the higher
diameter of the potassium ions, the volume of the potassium enriched volume is
increased. Resulting mechanical stress leads to a higher mechanical strength of the
part, like with thermal tempered glass. The process is conducted at high temperature
by immersing the glass part into a molten potassium salt (fig. 7).
Fig. 7: Scheme of chemical strengthening:
7a: before ion exchange (left)
7b: after ion exchange (right)
6. Modification by adding matter to the original surface (coating)
For the surface modification by coating, very different techniques are in use.
The field can be roughly distinguished into 4 sections.
6.1 Physical vapour deposition (PVD)
For this process the coating material is evaporated in high vaccum by high
temperature and condensated at the glass surface (fig. 8). The growing rate of the
layer can be controlled by special measurement equipment. Often more than one
coating are applied. So mirrors or antireflection coatings can be applied by a
sequence of coatings with high and low refraction index. PVD coatings are used for
antireflection coatings and metal oxide vaporized mirrors.
6.2 Sputtering
Sputtering is a high vacuum process as well, with a certain similarity to PVD.
In opposite to PVD, the coating material is not thermally evaporated, but released by
ion bombardment of a target. The matter, which is released from the target, is settling
on the glass surface (fig. 9). Sputtering is more versatile compared to PVD, it is e.g.
also possible to sputter materials with a very low vapour pressure.
Sputtering meanwhile became the standard procedure for low-E coatings for window
glass.
Fig. 8: Scheme of an Electron beam physical vapour deposition (EBPVD) plant
Fig. 9 Scheme of sputter deposition
6.3 Chemical Vapour deposition (CVD)
The CVD process is characterized by the fact, that a solid coating is formed
from one or more vaporized chemical precursors (fig. 10). Depending on the nature
of the chemical reaction, such a process may also be conducted at atmospheric
pressure.
A typical PVD process a atmospheric pressure is the hot-end coating of
container glass. Here a volatile, tin-organically substance is reduced at the hot glass
surface to tin oxide.
Fig. 10: Hot-end coating of container glass
6.4.1 Wet coatings.
For wet coatings, a paste or liquid is applied to the glass surface. By either a
chemical and / or physical process the wet layer is hardened to a coating. Depending
on the process, the coating needs an additional curing process to achieve the final
durability.
6.4.1.1 Inorganic coatings
Inorganic coatings are known under the name „ceramic coatings” or „Sol-Gel“.
Inorganic coatings are applied as fluid or paste. A first drying is typically conducted
by solvent evaporation. To achieve full hardness, these coatings need to be fired at a
high temperature, the upper temperature is typically limited by the properties of the
basic glass.
6.4.1.1.1 Ceramic coatings
Ceramic coatings are mostly a dispersion of a glass powder, which is melting
on the basic glass while curing. It is necessary that the transformation temperature of
the coating is below the transformation temperature of the basic glass. Colour effects
can be achieved with colouring metal salts. Ceramic paints give durable and robust
coatings.
A special case is the precipitation of precious metals (gold, platinum) on glass.
Such formulations are based on metal salts, which are reduced to the metal at firing.
Ceramic coatings have a wide field of application for flat glass as well as for
hollow glass. A typical application is the black print around the rim of a car
windshield. Here the gluing of the window shall be covered – not only for nice
appearance – but also to protect the glue from the harmful influence of sunlight.
6.4.1.1.2 Sol-gel
The classical sol-gel method gives inorganic coatings as well. These are
formed from a colloid by evaporation of the liquid phase. The solid phase of the
colloid is synthesized from a solution of precursors (fig. 11).
Fig. 11: Basic chemical reactions for Silica deposition by sol-gel
Usually oxide coatings can be generated with sol-gel, typical are coatings
made of silicon oxide.
The coatings are massively shrinking while the curing process, which is a
result of the loss of the liquid phase. This is why only thin layers are possible, thick
layers are cracking.
Sol-gel is often used for interference coatings.
6.4.1.2 Organic coatings
Glass can be decorated with organic paints as well. Based on usual industrial
coatings such coatings are modified for glass coating. One- ore two-component
systems or powder coatings are usual. Some important quality criteria for an organic
glass paint are:
o
good adhesion to glass (even under influence of water)
o
high gloss
o
high scratch resistance
o
good levelling
o
high transparency
Due to the easy application and curing, organic paints are often used for glass
decorations. The durability of ceramic coatings is generally not obtained by organic
coatings.
6.4.1.3 Hybrid coatings
Hybrid coatings were developed from sol-gels to overcome the limitations to
layer thickness as well as to improve chemical resistance. Hybrids contain inorganic
as well as organic matter. So the shrinking of the coating is reduced which makes
thicker coatings possible, compared to sol-gels. Due to the thicker layers, colorants
Fig. 12: SEM view on a broken glass, which has previously been coated with a sol-gel hybrid
can be introduced in the coating, so transparent or opaque decoration coatings are
possible. Typical decoration coatings have a thickness between 1 and 10 micron. So
hybrids are still thinner than organic coatings. That does not only result in economical
advantages, also the appearance profits from the thin layers (fig. 12).
7. Examples for application techniques with wet coatings
For the surface removing techniques, as well as for the vacuum techniques,
the process is rather defined by the technical procedure. For the wet coatings,
everything is possible, which leads to a success. In the following section, some
application techniques for wet coatings shall be introduced.
7.1 Standard techniques
Standard techniques for coating are dip coating (fig. 13), spray application
(fig. 14) and brush painting (fig. 15). Dip coating and spray coating can be
automated easily und are often used in mass production.
7.2 Felt pen
Application by felt pen has – compared to brush painting – the advantage, that
the painting can be done continuously, because the paint flow works automatically.
Lines, which fit to the width of the tip are easier to draw than with a brush
Fig 13: dip coating
Fig 14: spray coating
Fig 15: brush coating
7.3 Coating of bulk material
For bulk material a solvent based system can be applied. The bulk material (grains or
beads) are intensively mixed with the paint, so that the paint spreads equally over the
whole surface of the bulk material. After that, the solvent is blown out, so the paint is
fixed and can be cured afterwards (fig. 16). It is important, that the paint spreads
easily over the material and that the paint does not glue the material together.
Fig. 16: Paint application on glass beads
7.4 Inside-coating of tubes Innenbeschichtung von Röhren
An interesting
technique is the
inside coating of
tubes with a
homogenous
coating. This can be
done in analogy to a
dip coating process,
by filling the tubes
with paint and
running it out after
that. It is not trivial to
get homogenous
layers over the
length of the tube.
For solvent based
paints, a defined
ventilation of the
tube is helpful to fix
the coating (fig. 17).
Fig. 17: inside coating of glass tubes
7.5 Digital printing
For hybrid paints, two interesting digital printing techniques have been
developed.
7.5.1 Airbrush print
For airbrush printing, the paint is sprayed to the glass with digitally controlled
nozzles. Colour generation works by sequential application of the basic colours cyan,
magenta, yellow and black. The pictures are created pixel by pixel. The resolution is
42 dpi (dots per inch) (fig 18). The maximum size of the print is 3.5 x 6 metre.
Fig. 18: Print on glass with Ormojet Airbrush hybrid paint, printed on a TCG
Michelangelo printer
7.5.2 Ink-jet Print
The viscosity of the hybrid ink can be made so low, that it can be printed even
with an ink-jet printer (fig. 19). For ink-jet, a piezoelectric printing head can be used.
Often Epson heads are used, which make a resolution of 1440 dpi possible. The size
of a single ink droplet is 2 picolitre. With hybrid paints, prints with good resistance on
glass are possible. The ink can be coloured either with dies or with pigments. A white
ink is also available.
At the moment, different flat-bed plotters are offered, which allows prints on
glass with a size up to 2.5 x 3.5 m. For cylindrical hollow glass, the first prototypes
wave been successfully tested and we expect in future the development of printers
for printing on irregular shapes.
Fig. 19: Inkjet print on a glass plate with Ormojet hybrid ink, printed with a Technoplot
printer
8. Summary
Many properties of a glass object are determined by the properties of its
surface. This is why many techniques have been developed to change the original
glass surface to make the glass object either more decorative or to improve its
functionality. This paper gives a short overview of the most important techniques of
surface modification techniques on glass.
9. References
[1] Hüttentechnische Vereinigung der Deutschen Glasindustrie (HRSG): „Festigkeit
von Glas – Grundlagen und Prüfverfahren“ Frankfurt/Main, 1987
[2] H.K. Pulker: „Coatings on glass“, Amsterdam 1984
Prof. Dr.-Ing. Ferdinand Trier, Munich University of Applied Sciences, Laboratory for
Measurement Techniques Consultation for Glassing and Glasscoating, Lothstrasse
34, D-80335 München. e-mail: trier@fhm.edu
MEng. Dipl.-Ing. (FH) Ulrich Ranke, Ormo Print GmbH, Fuststrasse 8, D-80638
München. e-mail: ranke@lrz.fh-muenchen.de